The present invention relates to a linearized power amplifier based on active feedforward-type predistortion, and more particularly to linear power amplifiers for low distortion amplification of digitally modulated signals in cellular and satellite communication systems. More particularly, the invention relates to such a power amplifier for monolithic microwave integrated circuit (JAMIC) realization which includes an active feedforward-type predistorter and an output power amplifier and in which the predistorter amplifies and distorts the incoming signals for compensating the amplitude and phase nonlinearities of the output power amplifier and enhancing the PAE (power added efficiency) for linear amplification of incoming signals.
Digital modulation schemes are widely employed in various multi-carrier communication systems, such as wireless and satellite, for capacity improvement, better transmitted quality and high data rate transmission. In non-constant envelope digital modulation schemes, information is contained in both amplitude and phase of the modulated signals. To amplify such signals, linear amplifiers are required to prevent distorting the amplitude and phase characteristics which would degrade signal quality. In addition, linear amplifiers are also beneficial in amplifying multi-carrier signals simultaneously, in applications such as cellular basestations, without creating significant distortion. The advantage of employing linear amplifiers is that it reduces the number of amplifiers used as well as eliminates high power combiner chains, which is the conventional configuration for combining output power from several less linear amplifiers to limit distortion. This directly results in reducing size, complexity and cost of the overall amplification systems which is critical in applications such as satellite systems and cellular base stations. In addition to its low distortion characteristics, linear amplifiers should also attain high efficiency such that their DC power consumption can be minimized, resulting in higher performance, reliability and reduction of operating costs. Such features of linear amplifiers are highly desirable in all communication systems, and, in particular, cellular handsets in wireless systems wherein the overall size can be miniaturized and battery life, therefore the standby and talk time, can be significantly improved as a direct consequence of improved amplifier efficiency which is a primary concern in handset design.
To achieve linear amplification of non-constant envelope modulated signals in wireless communication systems, for instance, conventional amplifiers usually operate at certain output power backoff from saturated power in order to meet the linearity requirement. The tradeoff of this is a low PAE on the amplifiers since peak PAE is usually achieved near saturated output power level. By backing off from that operation, amplifiers could suffer as much as 30% to 40% reduction in PAE which has an adverse effect on DC power consumption, and, in particular, the battery life in cellular handsets.
The drawback of low efficiency on conventional linear amplifiers can be overcome by employing amplifier linearization techniques. Amplifier linearization techniques require the use of external circuitry to reduce distortion level at the output of amplifiers, thus allowing the amplifiers to operate into more efficient but nonlinear region, achieving high efficiency and good linearity simultaneously.
Common linearization techniques such as feedforward, predistortion, and feedback techniques have been disclosed in xe2x80x9cFeedfowardxe2x80x94An alternative approach to amplifier linearization,xe2x80x9d by T. J. Bennett et al., The Radio and Electronic Engineer, vol. 44, no. 5, pp. 257-262, May 1974; xe2x80x9cFeedforward linearization of 950 MHz amplifiers,xe2x80x9d by R. D. Stewart et al., IEEE Proceedings-H, vol. 135, no. 5, pp. 347-350, October 1988; U.S. Pat. No. 5,850,162 by Danielsons; xe2x80x9cAn automatically controlled predistorter for multilevel quadrature amplitude modulation,xe2x80x99 by J. Namiki, IEEE Trans. Commun., vol. COM-31, no. 5, pp. 707-712, May 1983; U.S. Pat. No. 4,465,980 by Huang et al.; U.S. Pat. No. 5,523,716 by Grebliunas et al.; xe2x80x9cAn MMAC C-Band FET feedback power amplifier,xe2x80x9d by A. K. Ezzeddine et al., IEEE Trans. Microwave Theory Tech., vol. MTT-38, no. 4, pp. 350-357, April 1990; xe2x80x9cLinearisation of microwave power amplifiers using active feedback networks,xe2x80x9d by F. Perez et al., Electron Lett., vol. 21, no. 1, pp. 9-10, January 1985; xe2x80x9cAn MMIC linearized amplifier using active feedback,xe2x80x9d by J. C. Pedro et al., 1993 IEEE MTT-S Dig., pp. 95-98; U.S. Pat. No. 5,886,572 by Myers et al.; U.S. Pat. No. 5,821,814 by Katayama et al. These techniques, however, usually involve either complex circuit configurations or experience possible stability problem which limit their practical applications in miniaturized cellular handsets.
Recently, predistorters with simpler configuration have been disclosed in xe2x80x9cA normal amplitude and phase linearizing technique for microwave power amplifiers,xe2x80x9d M. Nakayama et al., 1995 IEER MTT-S Dig., pp. 1451-1454; xe2x80x9cA novel series diode linearizer for mobile radio power amplifiers,xe2x80x9d by K. Yamauchi et al., 1996 IEEE MTT-S Dig., pp. 831-834; xe2x80x9cPassive FETMMIC linearizers for C, X and Ku-band satellite applications,xe2x80x9d A. Katz et al., 1993 IEEE MTT-S Dig., pp. 353-356; U.S. Pat. No. 5,191,338 by Katz et al. Although these predistortions achieve size reduction over conventional designs, they still require extra matching circuits, experience high loss and poor isolation which increase the difficulty in practical use. These are best illustrated by reference to the conventional miniaturized predistorter design shown in FIG. 1.
FIG. 1 shows a typical small-size passive type predistorter 104, including a nonlinearity generator 102, input matching circuit 101 and output matching circuit 103. The matching circuits usually include inductors which would take up significant area in an MMIC and increase the cost of the overall design. FIG. 2 shows a conventional configuration of a linearized power amplifier 206. The passive type predistorter 104, as illustrated in FIG. 1, is employed to compensate the nonlinear characteristics of a power amplifier 205. An isolator 201 and buffer amplifier 203 are required to compensate the poor reverse isolation and insertion loss of the predistorter 104, respectively. Matching circuits 202 and 204 are employed to match the input and output of the buffer amplifier 204 to the isolator 201 and the power amplifier 205, respectively. The disadvantages of this configuration include the requirement of extra matching circuits 202 and 204, buffer amplifier 203 and isolator 201, which increases the overall size of the linearized amplifier 206, and extra DC power consumption required by the buffer amplifier 203. The linearized amplifier 206 is, therefore, not practical for cellular handset applications which demands small size and low overall power consumption.
While prior art of low complexity passive type predistorter eliminates the use of bulky element. Such as power combiners, variable attenuators and phase shifters usually employed in conventional design, it includes nonlinearity generator with input and output matching. circuits. When cascaded with a power amplifier, the output matching of the predistorter duplicates the function of the input matching circuit of the amplifier. Since matching circuits usually include inductors which require large area to realize on MMICs, this increases the size of the chip and the colt of the overall design which are a critical concern for handset power amplifiers, and might not be feasible for volume production. In addition, conventional predistorters usually experience poor reverse isolation. Power amplifiers incorporating the predistorters require additional isolators to improve circuit isolation to avoid interaction between the predistorters and amplifier stages which would degrade overall circuit performance. Isolators usually comes in the form of individual module and would be unsuitable to add on a small circuit area that is available in cellular handsets. Predistorters shown in prior art are all passive in nature with insertion loss level ranges from 4 dB to 20 dB depending on the design. Extra buffer amplifiers are usually added to compensate the high insertion loss. The use of buffer amplifiers is of particular concern as that would increase overall DC power consumption. Even though the overall efficiency of the linearized amplifier is improved, the extra DC power requirement places extra strain on the battery life in handsets and such amplifier configuration is considered unsuitable for handset application. Furthermore, these known predistorters usually requires accurate tuning in order to obtain the expected efficiency improvement and therefore might not be practical in terms of mass production and ease to use.
In the above circumstances, it had been required to develop a novel linearized power amplifier free from the above problem.
Accordingly, it is an object of the present invention to provide a novel linearized power amplifier free from the above problems.
It is a further object of the present invention to provide a novel linearized power amplifier which offers linear power amplification with high efficiency and which are improved in size and DC power consumption.
The present invention provides a linear power amplifier consists of a driver stage, employing an active feedforward-type predistorter, connected in cascade with a final power stage. The active feedforward-type predistorter consists of an amplifier with a predistorter connecting between its input and output. This driver stage has opposite gain and phase characteristics to that of the final power stage and is used to predistort an input signal. When combined with the final power stage, the nonlinear gain and phase of the power stage are compensated and linearized by the driver stage, resulting in a linear power amplifier with low distortion amplification and high efficiency operation.
According to another aspect of the present invention, a tunable biasing circuit allows tuning on the active feedforward-type predistorter using an external DC voltage. By tuning the characteristics of the active feedforward-type predistorter, the predistorter can be employed to compensate various degree of nonlinearities of any power stage.
These and other aspects of the present invention as disclosed herein will become apparent to those skilled in the art after a reading of the following description of the preferred embodiment when considered with the drawings.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.